Channel access and network resource management in a wireless network that allows both Carrier Sense Multiple Access with Collision Detection (CSMA/CD) and Time Division Multiple Access (TDMA) can use a Media Access Control (MAC) superframe.
The traffic in the TDMA part is due to the frames being transmitted by the nodes in allocated time slots. The network traffic in the CSMA part satisfies the need for asynchronous communications, which are generated by management frames, requests for time-slot allocation, transmission of data frames, and retransmissions of failed data frames.
The superframe specifies when a node is to receive and transmit data frames from and to other nodes. The superframe allocates resources so that nodes do not transmit concurrently if the nodes can interfere with each other on the same channel.
In an ad-hoc network, clients can join and leave the network at will, which makes network resource management difficult and time consuming. Many application in such a wireless network use a client-server model where there is a server node (server) and one or more client nodes (clients). The server defines the resources used by all clients in the superframe. The server periodically transmits information in a beacon frame (beacon). The beacon is used to synchronize network operations.
Then, the clients can receive downlink traffic data frames from the server, or transmit uplink traffic data frames to the server at specific times as defined in the beacon frame.
Depending on the type of wireless network, the server can allocate time slots to each client for its uplink and downlink traffic. In some cases, the server can specify a shared interval for uplink traffic from multiple clients. In this case, one or more clients may try to access a channel randomly, or by using a protocol such as CSMA/CD, etc., and start transmitting data frames. That can result in multiple clients transmitting the data frames on the same channel concurrently causing collisions, which amounts to a loss of data and a waste of bandwidth.
The client-server model can be used when multiple clients regularly communicate with the server. In the prior art, the server has complete control over the timing and frequency allocations using the beacon. The server allocates time and channel resources to the clients as needed.
Because the communications according to the client-server model are tightly coupled, one problem arises when one or more clients are out of sync with the server due to a loss of the beacon. That can happen, for example, when clients encounter severe and persistent interference that makes it impossible to receive the beacon.
One way to deal with the interference problem is to use frequency hopping where the server and the clients periodically use channels with different frequencies. While it is useful to change the channel when some of the clients experience excessive interference, it is quite possible that the new channel also experiences interference.
Another problem that needs to be addressed is to preserve synchronization between the server and clients across different channels. In the prior art, the server uses a pre-defined hopping sequence of channels at regular intervals.
The client that loses the link with server can switch to the next channel in the sequence, and wait for the next beacon. After the client receives the beacon, the client synchronizes to the server.
If the client fails to receive the beacon for any reason, the client can lose communications with the server for an extended time period. This can happen in many time-critical applications, such as factory automation, commercial building controls, etc., where extended delays in communications are unacceptable. To make the matters worse, industrial environments inside buildings are less suited for wireless communications.
In the conventional TDMA based hopping, such as in Dust Nets, the server has complete control over the hopping sequence and the clients must follow the same sequence in order to communicate with the server. In other cases, for example the frequency agility mechanism in ZigBee, the personal area network (PAN) coordinator can move to another channel after the current channel becomes noisy, and all other nodes must follow it.
Therefore, it is desired to improve time slot and channel allocations in wireless networks.